Monday, June 07, 2010

No Domain-Driven Design in Rails?

The inevitable question came up: Do Rails apps and developers take on less complex problems and therefore don't require Domain-Driven Design?

There's another question that we can ask which mirrors an observation I've heard form notable Domain-Driven Design (DDD) experts: Do .NET developers use Domain-Driven Design for problems that are far too simple to justify Domain-Driven Design? And... is the Domain-Driven Design used in .NET pop culture really Domain-Driven Design in fact, or is it a lighter Domain-Driven Design that leans mostly on the DDD pattern language and remains somewhat naive about DDD? If these questions are indeed valid questions, and if they're being asked by leading DDD practitioners, then is the question about Rails and DDD a question rooted in valid presumptions, or is it coming from a perspective that stands on shaky ground to begin with?

Here's an admittedly-provocative question: Do the .NET developers who are wondering whether Rails is for lessor apps (as evidenced by the absence of explicit Domain-Driven Design) actually have a firm grasp of what Domain-Driven Design is, or are they merely using a pattern language and suffering the self over-estimation that is far too common in technology orthodocracies?

I think there's a missing piece in the inquiry. We're failing to ask whether Domain-Driven Design can be accomplished with the Active Record pattern, or can it only be achieved with the Domain Model pattern?

Furthermore: Is the Active Record pattern the only model object pattern used in Rails apps?

Rails is often about the part of the app that often deals with user interaction, or more specifically: user agent interaction. Do you need to use Domain-Driven Design for this part of your app? Maybe you do. Maybe you already do. Maybe you already use Domain-Driven Design for this part of your app but you're not yet aware that you're only using the DDD pattern language to name archetypes and layer supertypes in your design. After all, it's not like there's a lack of precedent for this in .NET apps "using" Domain-Driven Design. It might even be the most common form of DDD in .NET.

One of the most salient teachings to come away from Domain-Driven Design with is Partitioning - conceptual, physical, and logical. Partitioning is reflected in Domain-Driven Design's Aggregate Root pattern, its Repository Pattern, and its Bounded Context pattern, amongst others. The increasing attention given to eventual consistency and Command-Query Responsibility Separation (CQRS) in context of Domain-Driven Design also reflects partitioning and its effects and considerations.

Rails' - the framework - has absolutely nothing to say about partitioning. And its not supposed to. Partitioning is about a solution's topology and architecture. Rails is about building the user-interactive strata of an app. What you do behind the scenes is entirely out of Rails' hands. Rails itself - including its models - are a descriptive DSL over the abstractions that are over the infrastructure. You can make Rails archetypes as loosely or tightly coupled as you want (although you might be taking the Rails part of your app in the wrong architectural direction if you pursue extensive schema de-coupling, and you might need to be a pretty solid Rails hacker to do so).

That said, the Rails ecosystem has built a plethora of plugins for the framework that allow these architectural styles to be accommodated in a solution where Rails is in-play. In the wild, partitioning is one of the most obvious aspects of solutions that are front-ended by Rails.

Large-scale apps like Shopify, Gowalla, GitHub, Hulu, Scribd, Highrise, Yellow Pages, etc, don't get built without partitioning, and separating write from read operations. And you can't build a Rails app without modeling - whether you do modeling in your head, in code, or with diagrams (all are supported, bu the way). You can't build any of the non-public line of business Rails apps being built by organizations like Amazon, Electronic Arts, IBM, JP Morgan, NASA, Yahoo, Rackspace and a host of others, without tackling complexity at the heart of software design, and doing so in a way that insulates the non-complex part of your app with the complex part of your app, and dealing with them on their own terms without one tripping all over the other.

So, is the Active Record pattern sufficient for building Domain-Driven Design apps? Again, it goes back to how much you feel you should drive the whole topology and architecture of your whole solution from a framework that is intended to serve one aspect of solution architecture.

Domain-Driven Design is often trivialized to being merely a ubiquitous language (which is yet another DDD concept that itself is often trivialized into a meaninglessness that ignores other DDD precepts), entity classes, repository classes, service classes, and specification classes, and a smattering of other often-obscured concepts in the DDD heritage.

There's still something to learn from this form of DDD, but let's get serious for a moment: it's often just DDD as a pattern language, or "DDD Light" (now with 90% less real complexity!).

The average .NET app using Domain-Driven Design could, in my experience, be done for much less if done in the Rails ecosystem. Apps that use DDD inappropriately sometimes even end up needing more DDD because developers and designers have added insurmountable accidental complexity. There's nothing that contradicts DDD's essential message more than adding complexity to a solution!

Active Record, Rails, and Ruby, used in the parts of an app where its appropriate, don't contradict DDD. Without any question, it absolutely will change the pattern language used to describe the part of the app where you use Rails - but usually, that's all it does. If all you can see of Domain-Driven Design, however, is the pattern language, then you likely won't be able to use Domain-Driven Design even when you believe that you are using it.

There's little need for a Repository archetype in Rails. That said, Rails apps often do exhibit the essence of the repository pattern in the resource-oriented idioms (Aggregate Root, Repository, Specification, Context, Contours) that are ubiquitous in the Rails developer and architecture culture. Do you need to have a Repository class in order to be responsible about partitioning and to provide collection-oriented semantics for aggregate root access? For that matter, can you build a Domain-Driven Design app in programming language that doesn't have any notion of what a "class" is? If you can't have a Repository class can you still do Domain-Driven Design?

Do you ever hang data access operations off of a Repository class that aren't consistent with Repository or Aggregate Root semantics? Ever pull a lazy-loaded instance of Product from an OrderItem instance without going through a Product Repository? The question here isn't whether you've violated DDD in this case, but whether DDD's necessary constraints serve you well in these situations, and whether you should be trying to impose these constraints on this particular part of the whole architecture.

In my experience, there's as much Domain-Driven Design happening in the Rails world as in the .NET world. The big difference is that in the Rails world there's no social capital to be gained by speaking in the DDD pattern language in social gatherings of programmers trying to make their way in the professional world. Rails developers tend to just point to the game-changing web innovations and properties they've built and leave it at that.

One last thing about Domain-Driven Design and .NET: .NET developers tend to not recognize that using DDD as a pattern language is in essence a way to have an elaborated pattern language for Dependency Injection, as the design archetypes borrowed from DDD are usually either layer supertypes or archetypes of injected dependencies in static languages.

DDD Light is a side-effect of a programming paradigm where composition can largely only be achieved through Class-Oriented Programming. When the paradigm changes, and composition is something that the language itself can do, the propensity for DDD Light tends to dissipate. And that is why you tend to have less explicit mention of DDD in Rails' culture. DDD Light is as necessary in Rails as explicit, class-oriented composition and static dependency injection is. That's to say: it isn't (always). This was another topic briefly touched on by the Herding Code episode.

The necessity for Domain-Driven Design in fact and in practice in Rails is already well-established. Rails folks are just less-interested in talking about it in DDD terms, and not at all interested in DDD Light, as it's orthogonal to the paradigm - both socially and technically.

Rails - the culture and ecosystem - doesn't do DDD Light as the .NET culture and ecosystem does. That's about all we can really say about Rails and DDD.

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